[PATCH 9/9] perf probe: Try to use symbol table if searching debug info failed

From: Arnaldo Carvalho de Melo
Date: Fri Aug 21 2015 - 12:12:35 EST


From: Wang Nan <wangnan0@xxxxxxxxxx>

A problem can occur in a statically linked perf when vmlinux can be found:

# perf probe --add sys_epoll_pwait
probe-definition(0): sys_epoll_pwait
symbol:sys_epoll_pwait file:(null) line:0 offset:0 return:0 lazy:(null)
0 arguments
Looking at the vmlinux_path (7 entries long)
Using /lib/modules/4.2.0-rc1+/build/vmlinux for symbols
Open Debuginfo file: /lib/modules/4.2.0-rc1+/build/vmlinux
Try to find probe point from debuginfo.
Symbol sys_epoll_pwait address found : ffffffff8122bd40
Matched function: SyS_epoll_pwait
Failed to get call frame on 0xffffffff8122bd40
An error occurred in debuginfo analysis (-2).
Error: Failed to add events. Reason: No such file or directory (Code: -2)

The reason is caused by libdw that, if libdw is statically linked, it
can't load libebl_{arch}.so reliable.

In this case it is still possible to get the address from
/proc/kalksyms. However, perf tries that only when libdw returns
-EBADF.

This patch gives it another chance to utilize symbol table, even if
libdw returns an error code other than -EBADF.

After applying this patch:

# perf probe -nv --add sys_epoll_pwait
probe-definition(0): sys_epoll_pwait
symbol:sys_epoll_pwait file:(null) line:0 offset:0 return:0 lazy:(null)
0 arguments
Looking at the vmlinux_path (7 entries long)
Using /lib/modules/4.2.0-rc1+/build/vmlinux for symbols
Open Debuginfo file: /lib/modules/4.2.0-rc1+/build/vmlinux
Try to find probe point from debuginfo.
Symbol sys_epoll_pwait address found : ffffffff8122bd40
Matched function: SyS_epoll_pwait
Failed to get call frame on 0xffffffff8122bd40
An error occurred in debuginfo analysis (-2).
Trying to use symbols.
Opening /sys/kernel/debug/tracing/kprobe_events write=1
Added new event:
Writing event: p:probe/sys_epoll_pwait _text+2276672
probe:sys_epoll_pwait (on sys_epoll_pwait)

You can now use it in all perf tools, such as:

perf record -e probe:sys_epoll_pwait -aR sleep 1

Although libdw returns an error (Failed to get call frame), perf tries
symbol table and finally gets correct address.

Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxxxx>
Cc: Brendan Gregg <brendan.d.gregg@xxxxxxxxx>
Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxx>
Cc: He Kuang <hekuang@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Kaixu Xia <xiakaixu@xxxxxxxxxx>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Zefan Li <lizefan@xxxxxxxxxx>
Cc: pi3orama@xxxxxxx
Link: http://lkml.kernel.org/r/1440151770-129878-2-git-send-email-wangnan0@xxxxxxxxxx
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
---
tools/perf/util/probe-event.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index fe4941a94a25..f07374bc9c5a 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -705,9 +705,10 @@ static int try_to_find_probe_trace_events(struct perf_probe_event *pev,
}
/* Error path : ntevs < 0 */
pr_debug("An error occurred in debuginfo analysis (%d).\n", ntevs);
- if (ntevs == -EBADF) {
- pr_warning("Warning: No dwarf info found in the vmlinux - "
- "please rebuild kernel with CONFIG_DEBUG_INFO=y.\n");
+ if (ntevs < 0) {
+ if (ntevs == -EBADF)
+ pr_warning("Warning: No dwarf info found in the vmlinux - "
+ "please rebuild kernel with CONFIG_DEBUG_INFO=y.\n");
if (!need_dwarf) {
pr_debug("Trying to use symbols.\n");
return 0;
--
2.1.0

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